The proximity sensor can be designed to be incorporated into a monitoring tag that might be secured around the appendage of an individual, for example. The monitoring tags can include a location detection means for detecting if the individual has moved outside a designated area within which electromagnetic (e.g. radio-frequency or RF) signals can be exchanged between the monitoring tag and a base unit. The monitoring tags can also include a global positioning system (GPS) device that uses electromagnetic signals (e.g. GPS signals) to determine the location of the individual and to track and record their movement. The proximity sensor can also be designed to be incorporated into a portable tracking device that is carried or worn by the individual and which receives RF signals from a separate monitoring tag that is secured to the individual. The portable tracking device will normally include a GPS device that uses GPS signals to determine the location of the individual and to track and record their movement. As such, the tags and/or portable tracking devices can be worn by hospital patients, the elderly, children or offenders and can raise an alarm or notify an authorised person if they move into or outside a designated area.
It is possible to deliberately block the electromagnetic signals by placing an object such as a sheet of metal foil, for example, around the housing of the monitoring tag or the portable GPS tracking device. There is therefore a need for a proximity sensor to determine if the electromagnetic signals are being deliberately blocked in an attempt to incapacitate or disable the monitoring tag or portable GPS tracking device.